Efficiency analysis of a distillation collumn in cryogenic seperation of air

[Arjun Gopal (11CHE1063)]

The point raised during the lecture today on efficiency of distillation collumn, is an interesting problem. The tray efficiency is determined by the ratio of change in mole fraction of a particular component across a tray to the equilibrium change of its mole fraction(Murphree efficiency). As the temperature difference across the collumn is very low , the effiicency of each tray drops (the murphree efficiency is almost directly proportional to temperature difference). So unless we increase temperature difference, which result in huge increase in electricity costs we are stuck with this design.

Thus instead of a tray collumn, currently Linde employs a packed collumn design to decrease pressure drop and increase the efficiency.

[Utsav Gosalia(11CHE1066)]

A significant progress in air separation technology was made in the mid-eighties. For the first  time, structured packings were used in cryogenic rectification. Packed columns work according to a similar principle as sieve trays. The intensive contact between liquid and vapour required for the rectification takes place on the huge surface area of the packing material. Liquid flowing down becomes increasingly richer in oxygen, whereby the ascending vapour is enriched with nitrogen. The main benefits of

packed columns compared to tray sieves area lower pressure drop and consequently a lower power consumption for the air separation process. This also set the basis for a new process for argon separation.

[Bhushan Patil 11CHE1036]

A packed fractionating column is more efficient than an unpacked one because it provides a large surface area for heat exchange between the ascending vapor and the descending liquid. This allows for multiple, successive condenstations and distillations and produces better separation between liquids.